The present invention relates to an aeration apparatus for purifying contaminated water in a sewage treatment plant, such as a factory drainage treatment station, a municipal sewage treatment station, and the like. More particularly, the invention relates to an aeration apparatus having a casing which is provided with a passage for the liquid to be treated, an impeller disposed in the passage, and an oxygen-containing gas injection mechanism adapted for injecting the gas into the passage.
In the aeration apparatus of the type mentioned above, a gas-liquid recirculating flow is forcibly generated in an aeration tank so as to improve the aeration efficiency. Typical examples of conventional aeration apparatus of the type mentioned above are shown in FIGS. 1 and 2.
In the aeration apparatus shown in FIG. 1, an impeller 2 adapted to be driven by a motor 1 is disposed in a casing 3, so that the liquid flows upwardly or obliquely upwardly from the lower side as indicated by an arrow A. Air is introduced through an air supply pipe 4 to effect the aeration.
The aeration apparatus shown in FIG. 2 is of the type adapted to be kept afloat on the liquid surface by the buoyancy of a float 5. The liquid to be treated is made to flow as indicated by an arrow B from the upper to the lower side by an impeller 7 driven by a motor 6 and the aeration is effected by the air supplied through an air pipe 8.
These known apparatus themselves are effective but still suffer from a fundamental problem in that they cannot satisfactorily improve the aeration efficiency because the gas-liquid mixture is not circulated to the extremities of the aeration tank. Particularly, in the conventional apparatus shown in FIG. 1, there is a serious problem in that the aeration performance is undesirably lowered due to precipitation of the activated slurry which results from a flow velocity which is unduly low at the bottom of the aeration tank.
It has also been found that, in the aeration apparatus of the downward discharge type exemplified by the apparatus shown in FIG. 2, the aeration performance is seriously affected when the oxygen-containing gas injection port opening into the liquid passage in the apparatus is located too close to the upstream or downstream end of the impeller. For example, when the oxygen-containing gas is introduced at a position just upstream from the impeller, the suction side of the impeller is blocked by the gas if the gas supply rate is too large, so that the impeller fails to displace the water. In consequence, the gas is not drawn in and the flow direction of the liquid to be treated is undesirably reversed to the suction side.
In the case where the gas is supplied to the region just downstream from the impeller, the gas is gradually accumulated and will occupy the entire space in which the impeller is situated if the gas supply rate is too large. In consequence, the impeller becomes inoperative and the gas is not discharged from the discharge port. As a result, the liquid to be treated is undesirably displaced back toward the suction port due to the buoyancy of the gas bubbles. In both cases, the aeration performance is seriously deteriorated because of lack of recirculating flow of the gas-liquid mixture.